For starters, I live in Atlanta, Ga and I am not an HVAC pro so this is a serious question that I would like answered for one of my clients. I understand the premise of a zoned system and on the surface, it sounds like a good idea. Then I considered the fact that I have always told my customers that it is a bad idea to close off vents in rooms they are not in because of duct leakage. Also, the second law of thermodynamics simply states the hot goes to cold, so now the unheated areas are doing their best to rob warm air from the heated areas. Finally, if your zoning a single system, aren't you creating on oversizing issue because the unit (that was probably oversized in the first place) is now servicing a smaller are than what it was designed for?
I look forward to your input.
I live in the Dallas/Ft Worth area and am not an HVAC pro myself but I have talked to variety of reputable HVAC professionals on this topic and offer the following.
Strictly from an economic viewpoint, in the short term, zoned systems tend to have an installation price advantage over non-zoned systems on larger structures simply because less equipment can be better utilized over a larger area. However, in order to achieve better equipment utilization requires a more complicated control system which tends to create more maintenance issues in the long term. So, in my view, zoning provides the installer with a tool to reduce the first cost which is the name of the game to get the job. The long term costs of owning that sysem is pushed off into the future which I consider a bad idea.
Technically, I think your last question is addressed by zoning proponents with variable speed compressors. However, the slower the compressor speed, the higher the air coil temperature which affects the unit's ability to dehumidify the home adequately. Install HVAC equipment with warmer air coils in a home that has received a good job of air sealing in an area with high humidity in the summer and you have yourself a recipe for mold disaster in the home. This is a big deal in Dallas/Ft Worth and, I suspect, a bigger deal in Atlanta.
When it comes to dehumidifying an air tight home in a high humidity location, the coldest air coil is your best friend.
4 zones? What's your sq footage approx? Very surprising that you have so many zones. Variable speed system?
I'm retrofitting my home as we speak into 2 zones. Top floor, main floor. I have the option of a third with my controller but the third zone basement would have been 288 sq ft with 8 ft ceiling. So, going to save my third zone on the controller for any future relay fails on the board. It can happen. Enough about me. Lets talk about you.
How satisfied are you with your home's HVAC? Do you know if the home has _ever_ called for all the zones at once?
(Sorry to hear about your clients drama. Felt like I was watching Mike Holmes)
Answer to your initial thread question: zones are good. MUCH better than oversizing.
Cut and paste:
Also good is http://optimalbuilding.com/files/elephant_in_the_room.pdf which talks about HVAC for High Performance Home
Oversized equipment sucks from a comfort prospective, especially for gas furnaces. The heat comes on, blows you out of the room, then it's freezing before it cycles again. Furnaces do tend to be sized for CFM in the southern climates. A 45,000 BTU is good for 1200CFM, any more than 3 tons of A/C requires a bigger furnace.
Seems like we're sliding into politics here.
I read the study you are showing. http://www.fsec.ucf.edu/en/publications/html/FSEC-PF-328-97/
Figure 4 shows that 17 hours out of the day the oversized units used more electricity than the undersized.
Conclusion: There is an increase in annual cooling energy use for homes that have oversized systems.
Reading their conclusions, I don't see it being trivial that oversizing units puts excess demands of electrical infrastructure. This goes beyond costs. Why #1pay more for a larger unit and then #2 waste electricity?
Poorly trained HVAC companies multiplied by number of units installed per year times number of years in business. Every drop in the ocean counts. Everything counts.
From the sidelines, this is a draw. Your both right, if you live in Florida. North of the dehumidifier line, it's all about heat. Here in the mountains we don't need AC and contractors will tell you that the most cost effective and comfortable system is a HEAT pump with humidity controls. I am saving about $100 a month this winter because my 180% over-sized heat pump has a new thermostat that virtually eliminates the strip heat. About cooling, the new thermostat has a programmable fan only option (which runs in the slow mode) and a humidistat. My cooling cost was less than $20/month last summer. This spring we will add a hot water heat pump that will dehumidify the basement in addition to the hot water at about 1/3 of the resistance heater cost.
However, the cost effective part may soon disappear when the subsidies dry up and taxpayers get wise that electricity is not 100% efficient. How that factor got into energy modeling programs is a testament to the lobbyist in DC. How about somebody blogging on the cost benefits of taking the AC out of DC.